Abstract: While the throughput of Chinese Port is continuously increasing, the vessel traffic flow and density of vessels booming fast, the risk of waterway keeps risting, Therefore, research on the causes of ship traffic accidents. The research is based on 80 investigation reports on maritime accidents in Zhoushan Port, Ningbo. The accident model for analysis is constructed based on 70 of the 80 accidents and verified by studying the other 10 cases. The factors which cause accidents in the port is revealed and the causative chains that lead to accidents are identified. The measures to improve the safety in the port are recommended. This research is also an example of casual analysis application of Bayesian Network.